The present invention relates generally to memory elements and more specifically to the use of polycrystalline silicon as a non-volatile memory element.
For the storage of a large quantity of data in as small an amount of space as possible, the industry has turned towards semiconductor memory elements. A first group of elements comprises the read only memories (ROM) which are manufactured or delivered to the end user with specific ones and zeros stored in the memory locations. A second group comprises programmable read only memories (PROM) which are memory elements which may be programmed by the user to produce irreversibly ones or zeros where desired in the memory array. A third group of semiconductor memories comprises electrically alterable read only memories (EAROM) which are memory elements which may be switched between one and zero non-volatile states. A fourth class of semiconductor memories are random access memories (RAM) which generally differ from the electrically alterable read only memories in their speed of writing and erasing. Also, RAM's are generally volatile elements which lose their memory state when not biased.
Because of their versatility, a major development in the industry has been directed to electrically alterable read only memories and random access memories. One branch of this development has been research in amorphous chalcogenide memory elements. These amorphous memory elements are non-volatile and switch from a high to a low resistance state upon application of a voltage higher than a threshold voltage. They have generally been used in slow write electrically alterable read only memories. Their use as a non-volatile fast read and write random access memory is described in U.S. Pat. No. 4,199,692, wherein the speed of amorphous chalcogenide switching elements is increased by switching the element to a lower high resistance state and not into its low resistance state. The amorphous chalcogenide memory elements have generally been research and development tools and have not usually reached the production stage. Consequently much data has not been collected on the proper materials, the processing steps, and end-use limitations. Thus, although the amorphous chalcogenide memory devices seem to provide versatility of operation while minimizing surface area, they have not resulted in a proven device. There is concern that amorphous chalcogenide switching elements may not be able to operate at high temperatures. A need exists for a device which operates with the versatility of the amorphous chalcogenide memory elements using known materials and processes.
One of the most commonly used materials in semiconductor integrated circuits is silicon. The use of a high resistivity polysilicon resistor as a semiconductor switching element is shown by Tanimoto et al in U.S. Pat. No. 4,146,902. The polysilicon resistor had resistivity in the range of 10.sup.4 to 10.sup.7 ohm-centimeters and was designed for use as a programmable read only memory being switched irreversibly from a high to a low resistance value.
At the other end of the spectrum is a technique for trimming or modifying the resistance of polycrystalline silicon resistors as shown by Amemiya et al in U.S. Pat. No. 4,210,996. The technique shown requires the polycrystalline layer to have an impurity concentration higher than 1.times.10.sup.20 atoms per cubic centimeter. The trimming technique is effectively a current annealing since the polycrystalline silicon resistor is never driven to be switched to a low resistance state.